A contactless smart card is commonly used for security access or payment systems. Contactless smart cards generally include an antenna, in the form of an inductor, coupled to an integrated circuit (IC). The IC commonly includes a capacitor which forms a resonant circuit with the antenna. A card reader presents an alternating magnetic field that excites the inductor/capacitor resonant circuit, which in turn energizes and powers the IC. The IC can then perform one or more functions, such as transmitting a card number through the antenna to the card reader.
Many smart cards include an integrated circuit (IC) device. The IC device includes various circuits and a central processing unit (CPU) for performing operations on data, including operations on secret data (e.g., manipulating a private key). The CPU can be subjected to an analysis attack by an individual seeking to recover the secret data. For example, an attacker may try to deduce a secret key by measuring various circuit voltages through the IC device, single-stepping the CPU clock, then measuring the circuit voltages again to deduce the operation performed by the CPU and the operand manipulated during the operation.
Conventional solutions to protect against analysis attacks use two low frequency detectors to detect low clock frequencies. A first low frequency detector is connected to a clock pin of the IC chip. A second, low frequency detector is connected to an internal clock. The second low frequency detector is capable of detecting lower clock frequencies than the first low frequency detector. The second low frequency detector typically includes a custom cell delay line which consumes a large area on the IC die. This large footprint can preclude using the second low frequency detector on more than one clock on the IC device.